Upon further investigation, it looks like this plugin maybe just doesn’t like WordPress feeds. I’m now using the blog-in-blog plugin for WP-within-WP feeds, and it’s working beautifully.
I get this same error–or rather I get it about half the time, in some browsers but not others. I’m trying to pull a feed from my library website’s blog onto another page of the site, and that’s where the error shows up. It seems to be working fine for a Twitter feed that I’m pulling in on another post. So yeah, if there’s anyone who could shed some light on this, I’d be most grateful.